1. Field of the Invention
The present invention relates to a shift control system for an automatic transmission.
2. Description of the Prior Art
In an automatic transmission for an automotive vehicle, generally, engagement of a plurality of frictional elements provided therein is hydraulically changed to make a shift operation. In other words, the frictional elements are selectively, actuated to switch a power transmitting path to thereby automatically establish a desired shift stage from among a plurality of shift stages of different speed ratios.
If the switching operations of the frictional elements are not made at proper timing, a shift shock or an engine speed race may be undesirably produced because of "a time lag".
In order to deal with such a problem, it has been proposed to provide a timing valve in a hydraulic circuit of the transmission and to subject the timing valve to a pilot pressure which is adjusted for one frictional element so as to accomplish a desirable switching operation timing of another frictional element, as disclosed in Japanese Patent Public Disclosure (JP A) No. 61-45157, laid open to the public in 1986.
In the above publication, a 2-4 brake with a piston is disclosed as the one frictional element. The piston is engaged with both a releasing and an engaging chamber of a hydraulic control circuit so as to move by virtue of a pressure difference between the releasing and engaging chambers to thereby make a switching operation of the 2-4 brake between released and engaged positions. In this case, when the hydraulic pressure is discharged from the engaging chamber or introduced into the releasing chamber, the 2-4 brake is released.
However, in the above structure disclosed in the Japanese Patent Public Disclosure (JP A) 61-45157, the hydraulic fluid has to be provided for not only the one frictional element but also the other frictional element. As a result, the hydraulic pressure for the one frictional element may be temporarily depressed during the switching operation. This phenomenon occurs particularly in at a low temperature because of high viscosity of the hydraulic fluid. When the viscosity of the hydraulic fluid is high, the hydraulic fluid does, not flow fluently in the circuit because of an increase in flowing resistance of the circuit. As a result, it becomes difficult for the hydraulic fluid to be supplied to the other frictional element smoothly.
Therefore, the temporary depression of the hydraulic pressure in the one frictional element may cause "a time lag" in the switching action of the one to the other frictional element so that a shift shock, or engine speed racing, may be produced.